Aluminum foil (Al) is used extensively in the food packaging industry as a moisture and gas barrier. Because of foil's poor flex crack resistance, inability to form a hermetic seal, and cost, it is typically combined with other materials such as paper, polyethylene (PE) and oriented polypropylene (OPP) in multilayer structures. Applications include many dry food packages such as powdered drink mix pouches as well as non-packaging applications. Some common multi-layer structures include Paper/LDPE/AI/LDPE and OPP/primer/LDPE/AI/LDPE in which LDPE is low-density polyethylene and the paper or OPP provide stiffness and a surface for printing. The primer allows LDPE to bond to the OPP substrate; the first LDPE layer provides adhesion to the aluminum foil and the second acts as a sealing layer.
These structures are typically made by extrusion coating or lamination, which involves laying down a molten curtain of the polymer between the substrates moving at high speeds as they come into contact with a cold roll. Adhesion of polyethylene to foil can be accomplished by processing at high coating temperatures (300 to 330° C.) so that a portion of the polyethylene oxidizes. Oxidization of the LDPE creates polar species that provide moderate adhesion to the aluminum foil.
In many applications, rather than using a coated layer of LDPE as the sealant, a LDPE or LLDPE (linear low-density polyethylene) film is used. For example: Paper/LDPE/AI/LDPE/PE-film. The PE-film may be LDPE or LLDPE, but is typically LLDPE-based. A PE film has a number of advantages over a LDPE coating: For example, the PE-film can be made thicker than a coating, allowing for caulking of the seal interface. LLDPE-films provide higher seal strength. Blown films have better strength and tear resistance. Blown films provide greater stiffness. Blown films, processed at much lower temperatures than extrusion coating, have less taste and odor problems (associated with the oxidation of the LDPE).
LDPE is the predominant resin currently used for extrusion lamination. However, a substantial problem with using LDPE to extrusion laminate PE-films to aluminum foil is that the adhesion of the LDPE to aluminum foil “ages down” with time. Adhesion of LDPE to aluminum foil is only marginal to begin with, but over a time period of one to several weeks, the bond strength often declines to a level that is no longer functional for the application. One explanation is that the aging may be associated with secondary crystallinity of the LDPE. During the lamination process, the LDPE is quenched very quickly and little primary crystallization can occur. Over time, small “secondary” crystals may form. As PE crystallizes, it shrinks. Shrinkage can put a stress on bonds and reduce peel strength. The LDPE-AI bond is the weakest of the bonds in the structure (bonds of the LDPE to the PE-film are typically inseparable), and so its peel strength is reduced.
In adhering foil to non-polar polymer compositions, use of an additional polymeric composition as an adhesive or “tie” layer can be beneficial. Often, more polar polymeric materials adhere to foil more readily than less polar materials. However, polar polymers that adhere well to foil may not adhere well to non-polar polymers. Therefore, a tie layer preferably provides a balance of properties that allow it to adhere both to foil and to non-polar polymers.
Chemical primers are sometimes used to promote adhesion to non-polar substrates, but this adds costs and causes environmental concerns with solvent-based systems. Thus, it is desirable to adhere foil to non-polar polymer compositions without the use of primers.